Optical coherence tomography (OCT) is a cross-sectional, non-invasive imaging modality, that has application in diverse areas of medical imaging. In ophthalmology, OCT has been widely used for imaging the retina, choroid and anterior segment. Functional imaging of the blood velocity and vessel microvasculature is also possible. Fourier-domain OCT (FD-OCT) has recently attracted more attention because of its high sensitivity and imaging speed compared to time-domain OCT (TD-OCT) embodiment, which uses an optical delay line for mechanical depth scanning with a relatively slow imaging speed. The spectral information discrimination in FD-OCT is accomplished either by using a dispersive spectrometer in the detection arm (spectral domain or SD-OCT) or rapidly scanning a swept laser source (swept-source OCT or SS-OCT).
Compared to spectrometer-based FD-OCT, swept-source OCT (SS-OCT) has many advantages, including its robustness to motion artifacts and fringe washout, lower sensitivity roll-off and higher detection efficiency, etc. (see for example Choma et al. “Sensitivity Advantage of Swept Source and Fourier Domain Optical Coherence Tomography.” Optics Express 2003 11(18): 2183-2189). Many different approaches have been implemented to develop high-speed swept sources, including semiconductor optical amplifier (SOA) based ring laser designs (see for example Yun et al “High-speed optical frequency-domain imaging” Opt. Express 11:2953 2003 and Huber et al “Buffered Fourier domain mode locking: unidirectional swept laser sources for optical coherence tomography imaging at 370,000 lines/s,” Opt. Express 13, 3513 2005), and short cavity lasers (see for example Kuznetsov et al “Compact Ultrafast Reflective Fabry-Perot Tunable Lasers For OCT Imaging Applications,” Proc. SPIE 7554:75541F 2010) among others. SOA based ring laser designs have been practically limited to positive wavelength sweeps (increasing wavelength) because of the significant power loss that occurred in negative tuning. This has been attributed to four-wave mixing (FWM) in SOAs causing a negative frequency shift in intracavity light as it propagates through the SOA (Bilenca et al “Numerical study of wavelength-swept semiconductor ring lasers: the role of refractive-index nonlinearities in semiconductor optical amplifiers and implications for biomedical imaging applications,” Opt. Lett. 31: 760-762 2006)
A commercially available short cavity laser (Axsun Technologies Billerica, MA) in excess of 100 kHz has been reported (see for example Kuznetsov et al “Compact Ultrafast Reflective Fabry-Perot Tunable Lasers for OCT Imaging Applications,” Proc. SPIE 7554: 75541F 2010). Short cavity lasers enable a significant increase in sweep speeds over conventional swept laser technology because the time needed to build up lasing from spontaneous emission noise to saturate the gain medium is greatly shortened (R. Huber et al “Buffered Fourier domain mode locking: unidirectional swept laser sources for optical coherence tomography imaging at 370,000 lines/s,” Opt. Express 13: 3513 2005). However, the effective duty cycle of the bidirectional sweeping short cavity laser was limited to less than 50% because of the FWM effects mentioned above. The effective repetition rate of the laser is thus limited.
Several methods have been proposed to increase the effective repetition rates of SS-OCT systems including sweep buffering with a delay line, and multiplexing of multiple sources, thereby increasing the duty cycle of the laser. The method used to multiplex these sweeps together may include components that introduce orthogonal polarizations to the sweeps originating from different optical paths. Combining diverse polarizations at a polarization beamsplitter is a very light efficient way of transmitting the light to a single beam path.
Goldberg et al. demonstrated a ping-pong laser configuration for high-speed SS-OCT system that achieves a doubling of the effective A-line rate by interleaving sweeps of orthogonal polarization in the same cavity (see Goldberg et al “200 kHz A-line rate swept-source optical coherence tomography with a novel laser configuration” Proceedings of SPIE v.7889 paper 55 2011). This design is illustrated in FIG. 1. The paths from two semiconductor optical amplifiers SOAs were combined by a polarizing beam splitter (PBS) to generate light of orthogonal polarization states and are controlled precisely in time to double the effective duty cycle of the overall laser output. Each path has its own frequency selecting filter (101 and 102) for creating the swept sources. One path has a half waveplate to flip the polarization state of one path relative to the other. The output light is linearly polarized and only two-polarization states (horizontal and vertical as indicated by arrow 104 and circle 105) were demonstrated. The increased speed was used to acquire neighboring scans more quickly, with similar density to scans that would be acquired with a comparable speed had polarization diversity not been introduced.
Potsaid et al. demonstrated another method to double the effective repetition rate of a swept source laser by buffering and multiplexing the sweep of a single laser source (see Potsaid et al “Ultrahigh speed 1050 nm swept source/Fourier domain OCT retinal and anterior segment imaging at 100,000 to 400,000 axial scans per second” Opt. Express 18: 20029-20048 2010), as shown in FIG. 2. The laser sweep was split by a 60:40 fiber coupler and the original sweep from the 40 percent output side was directed to the 50:50 fiber coupler for multiplexing. A ˜1 km length of fiber is used to delay the sweep from the 60 percent output by one half of the sweep period such that a “copy” of the sweep can be combined with the original sweep during the time period when the laser is off. Polarization controls are used to match the polarization states of the two sweeps. This doubles the repetition rate of the sweep at the outputs labeled 2 and 3 in FIG. 2.
However, the long fiber spool will cause a significant birefringence to the laser output. While the understanding of these particular methods to enhance speed of OCT acquisition has been appreciated, the use of these systems to provide enhancements to various types of OCT measurements has not.
It is an object of the present invention to provide improved systems capable of generating pulses of multiple polarization states at a high repetition rate. In addition it is an object of the present invention to make use of the unique polarization properties of these sources to improve several types of OCT measurements.